Impact of polystyrene microplastics on <i>Daphnia magna</i> mortality and reproduction in relation to food availability

Abstract

Microplastics (MPs) in the environment continue to be a growing area of concern in terms of acute and chronic impacts on aquatic life. Whilst increasing numbers of studies are providing important insights into microparticle behaviour and impacts in the marine environment, a paucity of information exists regarding the freshwater environment. This study focusses on the uptake, retention and the impact of 2 µm polystyrene MPs in the freshwater cladoceran <i>Daphnia magna</i> in relation to food intake (algae <i>Chlorella vulgaris</i>), with MP size chosen to approximately match the cell size of the algae. <i>Daphnia</i> were exposed to varied concentrations of MPs and algae. When exposed to a single concentration of MPs <i>Daphnia</i> almost immediately ate them in large quantities. However, the presence of algae, even at low concentrations, had a significant negative impact on MP uptake that was not in proportion to relative availability. As MP concentrations increased, intake did not if algae were present, even at higher concentrations of MPs. This suggests that <i>Daphnia</i> are selectively avoiding eating plastics. Adult <i>Daphnia</i> exposed to MPs for 21 days showed mortality after seven days of exposure in all treatments compared to the control. However significant differences were all related to algal concentration rather than to MP concentration. This suggests that where ample food is present, MPs have little effect on adults. There was also no impact on their reproduction. The neonate toxicity test confirmed previous results that mortality and reproduction was linked to availability of food rather than MP concentrations. This would make sense in light of our suggestion that <i>Daphnia</i> are selectively avoiding eating microplastics.